International Journal of ChemTech Research CODEN (USA): IJCRGG ISSN: 0974-4290 Vol.8, No.8, pp 121-133, 2015 Adaptive Control of Rikitake Two-Disk Dynamo System Sundarapandian Vaidyanathan* R & D Centre, Vel Tech University, Avadi, Chennai, Tamil Nadu, India Abstract: Chaos theory has a manifold variety of applications in science and engineering. The frequent and irregular reversals of the earth s magnetic field has motivated a number of research studies involving electrical currents within the earth s molten core. One of the first such nonlinear models that exhibited the frequent and irregular reversals of the earth s magnetic field was Rikitake s two-disk dynamo system (1958). Rikitake two-disk dynamo system is a chaotic system that predated the pioneering work of Lorenz (1963). In this paper, we describe the dynamic equations and qualitative properties of the Rikitae two-disk dynamo chaotic system. We also derive new results for the adaptive control of the Rikitake two-disk dynamo chaotic system. MATLAB plots have been depicted to illustrate the phase portraits of the Rikitake two-disk dynamo chaotic attractor and the global chaos control of the Rikitake two-disk dynamo chaotic systems via adaptive control method. Keywords: Chaos, chaotic systems, chaos control, earth s magnetic field, electrical currents, Rikitake dynamo system, two-disk model, nonlinear model, adaptive control, stability. 1. Introduction A dynamical system is called chaotic if it satisfies the three properties: boundedness, infinite recurrence and sensitive dependence on initial conditions [1-2]. Chaos theory investigates the qualitative and numerical study of unstable aperiodic behaviour in deterministic nonlinear dynamical systems. In 1963, Lorenz [3] discovered a 3-D chaotic system when he was studying a 3-D weather model for atmospheric convection. After a decade, Rössler [4] discovered a 3-D chaotic system, which was constructed during the study of a chemical reaction. These classical chaotic systems paved the way to the discovery of many 3-D chaotic systems such as Arneodo system [5], Sprott systems [6], Chen system [7], Lü-Chen system [8], Cai system [9], Tigan system [10], etc. Many new chaotic systems have been also discovered in the recent years like Sundarapandian systems [11, 12], Vaidyanathan systems [13-43], Pehlivan system [44], Pham system [45], etc. Recently, there is significant result in the chaos literature in the synchronization of physical and chemical systems. A pair of systems called master and slave systems are considered for the synchronization process and the design goal is to device a feedback mechanism so that the trajectories of the slave system asymptotically track the trajectories of the master system. Because of the butterfly effect which causes exponential divergence of two trajectories of the system starting from nearby initial conditions, the synchronization of chaotic systems is seemingly a challenging research problem. In control theory, active control method is used when the parameters are available for measurement [46-65]. Adaptive control is a popular control technique used for stabilizing systems when the system parameters are unknown [66-79]. There are also other popular methods available for control and synchronization of systems such as backstepping control method [80-86], sliding mode control method [87-98], etc.
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 122 Recently, chaos theory is found to have important applications in several areas such as chemistry [99-107], biology [108-125], memristors [126-128], electrical circuits [129], etc. The frequent and irregular reversals of the earth s magnetic field has motivated a number of research studies involving electrical currents within the earth s molten core. One of the first such nonlinear models that exhibited the frequent and irregular reversals of the earth s magnetic field was Rikitake two-disk dynamo system [130]. Rikitake two-disk dynamo system (1958) is a classical chaotic system that predated the pioneering work of Lorenz (1963). First, this research paper details the dynamic equations of the Rikitake two-disk dynamo system [130] and discusses its qualitative properties. This paper also derives new results for the global chaos control of the Rikitake two-disk dynamo system via adaptive control method. MATLAB plots are shown to depict the phase portraits and global chaos control of the Rikitake two-disk dynamo system. 2. Rikitake Two-Disk Dynamo Chaotic System Rikitake two-disk dynamo chaotic system [130] is governed by the system model ìx& 1 =- ax1+ xx 2 3 íx& 2 =- ax2 + x1( x3 -b) (1) îx& 3= 1-xx 1 2 where x 1, x 2, x 3 are the states and a, bare constant positive parameters. The parameter a represents the resistive dissipation and the parameter b represents the difference in the angular velocities of the two disks. We note that the Rikitake two-disk dynamo chaotic system (1) has the same number of terms as the Lorenz chaotic system, but with one additional nonlinearity. The Rikitake two-disk dynamo system (1) is chaotic when the system parameters are chosen as a= 1, b= 1 (2) For numerical simulations, we take the initial conditions x (0) = 1.0, x (0) = 0, x (0) = 0.8 (3) 1 2 3 Figure 1 shows the 3-D phase portrait of the Rikitake two-disk dynamo system (1). Figures 2-4 show the 2-D projections of the Rikitake two-disk dynamo system on the ( x1, x2), ( x2, x3) and ( x1, x3) planes, respectively. Figure 1. The 3-D phase portrait of the Rikitake two-disk dynamo system
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 123 Figure 2. The 2-D projection of the Rikitake two-disk dynamo system on the ( x1, x2) plane Figure 3. The 2-D projection of the Rikitake two-disk dynamo system on the ( x2, x3) plane
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 124 Figure 4. The 2-D projection of the Rikitake two-disk dynamo system on the ( x1, x 3) plane The Lyapunov exponents of the Rikitake two-disk dynamo system (1) are numerically found as L === 0.12749, L 0, L - 2.12704 (4) 1 2 3 From the LE spectrum (4), it is immediate that the Rikitake two-disk dynamo system (1) is a chaotic system and the Maximal Lyapunov Exponent (MLE) of the Rikitake dynamo system (1) is L 1 = 0.12749. Since the sum of the Lyapunov exponents in (4) is negative, it follows that the Rikitake two-disk dynamo system (1) is dissipative. Also, the Lyapunov dimension of the Rikitake two-disk dynamo system (1) is derived as D L L + L = + = L 1 2 2 2.0599 3 3. Global Chaos Control of the Rikitake Two-Disk Dynamo Chaotic System via Adaptive Control Method In this section, we use adaptive control method to achieve global chaos control of the Rikitake two-disk dynamo chaotic system with unknown parameters. We use Lyapunov stability theory [131] to prove the main adaptive control result derived in this section. Thus, we consider the controlled Rikitake two-disk dynamo dynamics given by ìx& 1 =- ax1+ x2x3 + u1 íx& 2 =- ax2 + x1( x3- b) + u2 (6) îx& 3 = 1- x1x2 + u3 In ((6), x1, x2, x3are the states of the Rikitake two-disk dynamo system and abare, unknown parameters. The design of the control goal is to find nonlinear feedback controls u1, u2, u3using estimates of the unknown system parameters so as to regulate the states x1, x2, x3of the Rikitake two-disk dynamo chaotic system (6) to constant values or set-point controls given by a1, a2, a3, respectively. Thus, the output regulation errors are defined by ìe1() t = x1() t -a1 íe2() t = x2() t -a2 (7) îe3() t = x3() t -a3 (5)
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 125 The error dynamics is obtained as ìe& 1 =- a( e1+ a1) + ( e2 + a2)( e3 + a3) + u1 íe& 2 =- a( e2 + a2) + ( e1+ a1)( e3+ a3- b) + u2 (8) îe& 3 = 1 -( e1+ a1)( e2 + a2) + u3 We consider the adaptive controller defined by ìu1= at ˆ( )( e1+ a1) -( e2+ a2)( e3+ a3) -ke 1 1 íu ˆ 2= at ˆ( )( e2+ a2) -( e1+ a1)( e3+ a3-bt ( )) -ke 2 2 (9) u3=- 1 + ( e1+ a1)( e2+ a2) -ke î 3 3 where k 1, k 2, k 3 are positive gain constants. Substituting (9) into (8), we get the closed-loop error dynamics as ìe& 1=-[ a- at ˆ( )]( e1+ a1) -ke 1 1 íe& ˆ ˆ 2=-[ a- a( t)]( e2+ a2) -[ b- b( t)]( e1+ a1) -k2e2 (10) e& 3=-ke î 3 3 We define the parameter estimation errors as ìe () ˆ a t = a-at () í (11) e() ˆ î b t = b-bt () Using (11), the closed-loop system (10) can be simplified as ìe& 1= - ea( e1+ a1) -ke 1 1 íe& 2=- ea( e2+ a2) - eb( e1+ a1) -ke 2 2 (12) îe& 3=-ke 3 3 Differentiating (11) with respect to time, we get ì e& () ˆ a t =-at &() í (13) e() ˆ& î & b t =-bt () Next, we consider the candidate Lyapunov function defined by 1 2 2 2 2 2 V( e1, e2, e3, ea, eb) = ( e1 + e2 + e3 + ea + eb) (14) 2 Differentiating (14) along the trajectories of (12) and (13), we get the following dynamics 2 2 2 V& =-ke 1 1 -k2e2 - k3e3 + e é 1( 1 1) 2( 2 2) ˆ ˆ a - e e + a - e e + a - a& ù e é & + b - e2( e1+ a1) -bù (15) ë û êë úû In view of (15), we take the following parameter update law: ì a& ˆ() t =- e1( e1+ a1) - e2( e2 + a2) í (16) & îbˆ() t =- e2( e1+ a1) Next, we state and prove the main result of this section. Theorem 1. The adaptive control law (9) and the parameter update law (16) achieve global chaos control of the Rikitake two-disk dynamo system (6) by regulating the states x1(), t x2(), t x3() t so as to track the set-point controls a1, a2, a3, respectively, where k 1, k 2, k 3 are positive gain constants. Proof. The result is proved using Lyapunov stability theory [131]. The quadratic Lyapunov function V defined by (14) is positive definite on R 5. Substituting the parameter update law (16) into (15), we get the time derivative of V as
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 126 V & =-ke -k e -k e 2 2 2, 1 1 2 2 3 3 (17) which is negative semi-definite on R 5. Thus, by Barbalat s lemma in Lyapunov stability theory [131], it follows that the closed-loop error dynamics (12) is globally exponentially stable. Hence, we have shown that adaptive control law (9) and the parameter update law (16) achieve global chaos control of the Rikitake two-disk dynamo system (6) by regulating the states x1(), t x2(), t x3() t so as to track the set-point controls a1, a2, a3, respectively. This completes the proof. 4. Numerical Simulations -8 We use the classical fourth-order Runge-Kutta method with step-size h = 10 to solve the system of differential equations (6) and (7), when the adaptive control law (10) is implemented. We take the parameter values of the Rikitake two-disk dynamo chaotic systems as in the chaotic case, viz. a= 1, b= 1 (18) We take the gain constants as k1 = 6, k2 = 6, k3 = 6 (19) We take the initial values of the Rikitake dynamo system (6) as x1(0) = 7.5, x2(0) = 17.2, x3(0) = 23.4 (20) We take the set-point controls as a1 = 1, a2 = 2, a3 = 3 (21) We take the initial values of the parameter estimates as aˆ(0) = 3.4, bˆ (0) = 7.6 (22) Figures 5-7 show the output regulation of the Rikitake dynamo chaotic system (6). Figure 8 shows the time-history of the regulation errors e1, e2, e3. Figure 5. Output regulation of the state x () t 1
Sundarapandian Vaidyanathan /Int.J. ChemTech Res. 2015,8(8),pp 121-133. 127 Figure 6. Output regulation of the state x () t 2 Figure 7. Output regulation of the state x () t 3 Figure 8. Time-history of the regulation errors e1(), t e2(), t e3() t
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